The present invention relates to a power conversion apparatus for converting an AC electric power into a DC electric power or converting a DC electric power into an AC electric power, and a power conversion system. More particularly, the invention relates to a power conversion apparatus which includes an arm configured by connecting one or plural unit converters in series with one another, and thus is suitable for converting an electric power, and a high-voltage DC transmission system.
In recent years, a technique for converting an AC electric power into a DC electric power or converting a DC electric power into an AC electric power has been used in many cases. As far as this sort of apparatus, an apparatus which utilizes a unit converter is known. This apparatus is such that plural unit converters are connected in series with one another, and a semiconductor switching device is operated for an energy storage element in each of the unit converters, thereby carrying out an electric power converting operation. A power conversion apparatus which can withstand a high voltage is relatively, readily obtained.
As far as this power conversion apparatus, a circuit configuration is known in which plural series circuits each composed of an arm composed of a series circuit of one or plural unit converters, and a transformer winding are connected in parallel with one another. This power conversion apparatus, for example, is described in JP-A-2010-233411.
The circuit configuration described above is a type of multilevel converter. Thus, with the circuit configuration concerned, a voltage equal to or higher than a withstanding voltage of the switching device can be outputted by using an ON/OFF controllable power semiconductor device such as an Insulated-Gate Bipolar Transistor (IGBT), a Gate Turn-Off Thyristor (GTO), or a Gate-Commutated Thyristor (GCT) or the like.
It is noted that in this specification, a circuit system of the power conversion apparatus disclosed in JP-A-2010-233411 is referred to as a Zero-Sequence Cancelling Modular Multilevel Converter (ZC-MMC).